In chairs used for office work or the like, a seat that supports a seated person's buttock, and a backrest that supports a seated person's back are attached to upper parts of leg posts. A flexible sensation of contact, holding performance for the seated person's back, and the like are required for the backrest. Backrests of chairs that respond to such requirements have been contrived for a long time (for example, refer to Patent Documents 1 to 5).
In a backrest for a chair described in Patent Document 1, side edge support parts of which the forward-backward thickness is greater than that of a back plate part are integrally formed on both left and right sides of a substantially rectangular back plate part, and a fitting hole into which a backrest post is fitted provided at a lower edge of each of the side edge support parts. Left and right edges of the backrest are combined with the left and right backrest posts in the portions of the fitting hole portion. In the case of this backrest for a chair, the side edge support parts with large forward and backward thickness are continuously in the upward-downward direction on both the left and right sides of the back plate part. Hence, if the load of a seated person's upper body is input, rigidity can be maintained by the left and right side edge support part while flexibly deforming the back plate part, and the user's upper body may be held so as to be wrapped therein.
Additionally, in backrests for chairs described in Patent Documents 2 and 3, an outer peripheral region of a substantially rectangular back plate part is surrounded by a frame part with higher rigidity than the back plate part, and, left and right lateral side portions of the frame part are coupled to backrest posts. In the case of these backrests for chairs, the rigidity of peripheral region part can be maintained by the frame part while flexibly deforming the back plate part.
Additionally, in a backrest for a chair described in Patent Document 4, a back plate made of synthetic resin is attached to a backrest frame, and the back plate are provided with a plurality of net-shaped openings. In the case of this backrest, a flexible sensation of contact and holding performance are obtained by guaranteeing support rigidity by the backrest frame and allowing the back plate supported by the backrest frame to be easily elastically deformed by the net-shaped openings.
However, since the backrest for a chair described in Patent Document 4 is made to have a structure including the backrest frame and the backrest frame having the back plate that is a separate component, the number of components is large, and assembly work is apt to become complicated.
Meanwhile, in the backrests for a chair described in Patent Documents 2 and 5, the problem that the number of components increase is solved by integrally providing the frame part with high strength and the back plate part allowing flexible bending deformation in a load support plate made of synthetic resin.
Specifically, in the backrests described in Patent Documents 2 and 5, a thick frame part of which the inside is reinforced by ribs or the like is formed at left and right side edge parts, an upper side part, and a lower side part of the substantially rectangular load support plate made of synthetic resin. Additionally, a thin back plate part is formed in an inside region of the frame part, and a plurality of elongated hole-shaped openings that are long in the upward-downward direction are formed in a substantially entire region of the back plate part. In the case of this backrest, support rigidity is guaranteed by the thick frame part, and a flexible sensation of contact and holding performance are obtained by the thin back plate part having the plurality of openings.